The present invention relates to a process for producing a self-dispersing and salt-sensitive polymer having such characteristic properties that it is soluble or dispersible in a usual tap water but is insoluble in water containing not less than 0.2% by weight of neutral inorganic salts (such as sodium chloride, potassium chloride, sodium sulfate and potassium sulfate).
It is known that most polymer emulsions are produced by an emulsion polymerization process and that an emulsifier used in the process will impair the capacity and properties of the film after drying. Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") Nos. Sho 58-136647 and Sho 63-280702 disclose processes for solving this problem, which comprise conducting the polymerization in an organic solvent, distilling off the organic solvent and adding water to the residue to conduct the self-dispersion. However, these processes are for the production of essentially water-insoluble polymers, and as described in Examples given in the specifications, films prepared from such a polymer emulsion are not dissolved or dispersed even when they are immersed in water.
On the other hand, it has been known that when an inorganic salt which is an electrolyte is added to an aqueous solution of a water-soluble polymer, the polymer is salted out to form a precipitate. On the other hand, it has been known that salts are contained in human body fluids such as urine in an amount of at least about 0.5% by weight (hereinafter referred to as "%", otherwise specified). It is supposed, therefore, that when a non-woven fabric or paper is bonded by means of a binder comprising a polymer which is soluble or dispersible in tap water but insoluble in an aqueous solution containing salts in an amount of about 0.5% by weight, the resulting product will have a sufficient strength when it is brought into contact with a body fluid, but the product will be easily dispersed in a flush toilet. Under these circumstances, investigations are made on the use of the salt-sensitive polymer as a binder for various products to be thrown into the flush toilet or the like. Among the salt-sensitive polymers, anionic polymers such as sodium salts of polyacrylic acid and carboxymethyl cellulose are less sensitive to an increase in the concentration of an electrolyte in an aqueous solution. Such anionic polymers become insoluble in, for instance, an aqueous solution of common salt when the common salt concentration is increased to as high as 4 to 5% or above. Although J. P. KOKAI No. Sho 50-52371 discloses an alkali cellulose ether sulfate, the difference between the solubility of this sulfate in tap water and that in 0.5% aqueous common salt solution is not clearly described in this patent. Moreover, since the alkali cellulose ether sulfate is prepared by sulfating a cellulose derivative such as ethyl cellulose, the polymer used as the starting material is relatively expensive.
As for a salt-sensitive polymer for solving the foregoing problems, J. P. KOKAI No. Sho 63-139906 discloses a partially neutralized copolymer of (meth)acrylic acid and a (meth)acrylic ester as the polymer soluble in tap water but insoluble in an aqueous solution of common salt having a concentration of not less than 0.5%. However, such a partially neutralized copolymer has a high adhesion. J. P. KOKAI No. Hei 3-174417 discloses a partially neutralized copolymer of (meth) acrylic acid and an aryl or cycloalkyl (meth)acrylate; or a partially neutralized copolymer of (meth)acrylic acid and a styrene derivative which copolymers have no problem of the adhesion. However, non-woven fabrics or papers prepared by using such a polymer as the binder do not have satisfactory strength and permeability to the body fluid practically demanded when they are wetted with the body fluid. Namely, when the strength of the non-woven fabrics or papers is sufficiently increased, the water-repellency of the polymer becomes excessively high and the permeability to the fluid is reduced. On the contrary, when the permeability to the fluid is satisfactorily increased, the strength of a film or the like prepared from the polymer becomes insufficient. Salt-sensitive binders disclosed in J. P. KOKAI Nos. sho 63-139906 and Hei 3-174417 are used in the form of a solution in a mixed solvent of an organic solvent such as acetone or methanol with water. When such a binder is used as the binder for non-woven fabrics or papers, it poses problems such as smell and safety, e.g. fire.